Monday, May 15, 2017

Today we are excited to announce the launch of a new JavaScript execution pipeline for V8 5.9 that will reach Chrome Stable in M59. With the new pipeline, we achieve big performance improvements and significant memory savings on real-world JavaScript applications. We’ll discuss the numbers in more detail at the end of this post, but first let’s take a look at the pipeline itself.

The new pipeline is built upon Ignition, V8’s interpreter, and TurboFan, V8’s newest optimizing compiler. These technologies shouldbefamiliar to those of you who have followed the V8 blog over the last few years, but the switch to the new pipeline marks a big new milestone for both.

For the first time, Ignition and TurboFan are used universally and exclusively for JavaScript execution in V8 5.9. Furthermore, starting with 5.9, Full-codegen and Crankshaft, the technologies that served V8 well since 2010, are no longer used in V8 for JavaScript execution, since they no longer are able to keep pace with new JavaScript language features and the optimizations those features require. We plan to remove them completely very soon. That means that V8 will have an overall much simpler and more maintainable architecture going forward.

A Long Journey

The combined Ignition and TurboFan pipeline has been in development for almost 3½ years. It represents the culmination of the collective insight that the V8 team has gleaned from measuring real-world JavaScript performance and carefully considering the shortcomings of Full-codegen and Crankshaft. It is a foundation with which we will be able to continue to optimize the entirety of the JavaScript language for years to come.

The TurboFan project originally started in late 2013 to address the shortcomings of Crankshaft. Crankshaft can only optimize a subset of the JavaScript language. For example, it was not designed to optimize JavaScript code using structured exception handling, i.e. code blocks demarcated by JavaScript’s try, catch, and finally keywords. It is difficult to add support for new language features in Crankshaft, since these features almost always require writing architecture-specific code for nine supported platforms. Furthermore, Crankshaft’s architecture is limited in the extent that it can generate optimal machine code. It can only squeeze so much performance out of JavaScript, despite requiring the V8 team to maintain more than ten thousand lines of code per chip architecture.

TurboFan was designed from the beginning not only to optimize all of the language features found in the JavaScript standard at the time, ES5, but also all the future features planned for ES2015 and beyond. It introduces a layered compiler design that enables a clean separation between high-level and low-level compiler optimizations, making it easy to add new language features without modifying architecture-specific code. TurboFan adds an explicit instruction selection compilation phase that makes it possible to write far less architecture-specific code for each supported platform in the first place. With this new phase, architecture-specific code is written once and it rarely needs to be changed. These and other decisions lead to a more maintainable and extensible optimizing compiler for all of the architectures that V8 supports.

The original motivation behind V8’s Ignition interpreter was to reduce memory consumption on mobile devices. Before Ignition, the code generated by V8’s Full-codegen baseline compiler typically occupied almost one third of the overall JavaScript heap in Chrome. That left less space for a web application’s actual data. When Ignition was enabled for Chrome M53 on Android devices with limited RAM, the memory footprint required for baseline, non-optimized JavaScript code shrank by a factor of nine on ARM64-based mobile devices.

Later the V8 team took advantage of the fact that Ignition’s bytecode can be used to generate optimized machine code with TurboFan directly rather than having to re-compile from source code as Crankshaft did. Ignition’s bytecode provides a cleaner and less error-prone baseline execution model in V8, simplifying the deoptimization mechanism that is a key feature of V8’s adaptive optimization. Finally, since generating bytecode is faster than generating Full-codegen’s baseline compiled code, activating Ignition generally improves script startup times and in turn, web page loads.

By coupling the design of Ignition and TurboFan closely, there are even more benefits to the overall architecture. For example, rather than writing Ignition’s high-performance bytecode handlers in hand-coded assembly, the V8 team instead uses TurboFan’s intermediate representation to express the handlers’ functionality and lets TurboFan do the optimization and final code generation for V8’s numerous supported platforms. This ensures Ignition performs well on all of V8’s supported chip architectures while simultaneously eliminating the burden of maintaining nine separate platform ports.

Running the Numbers

History aside, now let’s take a look at the new pipeline’s real-world performance and memory consumption.

The V8 team continually monitors the performance of real-world use cases using the Telemetry - Catapult framework. Previously in this blog we’ve discussed why it’s so important to use the data from real-world tests to drive our performance optimization work and how we use WebPageReplay together with Telemetry to do so. The switch to Ignition and TurboFan shows performance improvements in those real-world test cases. Specifically, the new pipeline results in significant speed-ups on user interaction story tests for well-known websites:

Reduction in time spent in V8 for user interaction benchmarks

Although Speedometer is a synthetic benchmark, we’ve previously uncovered that it does a better job of approximating the real-world workloads of modern JavaScript than other synthetic benchmarks. The switch to Ignition and TurboFan improves V8’s Speedometer score by 5%-10%, depending on platform and device.

The new pipeline also speeds up server-side JavaScript. AcmeAir, a benchmark for Node.js that simulates the server backend implementation of a fictitious airline, runs more than 10% faster using V8 5.9.

Improvements on Web and Node.js benchmarks

Ignition and TurboFan also reduce V8’s overall memory footprint. In Chrome M59, the new pipeline slims V8’s memory footprint on desktop and high-end mobile devices by 5-10%. This reduction is a result of bringing the Ignition memory savings that have been previously covered in this blog to all devices and platforms supported by V8.

These improvements are just the start. The new Ignition and TurboFan pipeline paves the way for further optimizations that will boost JavaScript performance and shrink V8’s footprint in both Chrome and in Node.js for years to come. We look forward to sharing those improvements with you as we roll them out to developers and users. Stay tuned.

Wednesday, May 3, 2017

The team behind Atom, a text editor based on the Electron framework, recently published an article detailing significant improvements to start-up time, owing big parts of the gains to the usage of V8's custom start-up snapshot. Awesome!

Native Electron apps, including Atom, leverage Chromium to display a GUI and Node.js as an execution environment, both of which respectively embed V8 to run JavaScript. This allows Electron apps to take advantage of V8 snapshots to quickly initialize a previously serialized heap for faster startup. Electron developers have even released electron-link, a convenience library for setting up this feature, which Atom heavily relies on for its performance optimizations.

We announced V8's support for custom start-up snapshot more than a year ago. With v8::V8::CreateSnapshotDataBlob, embedders can provide an additional script to customize a start-up snapshot. New contexts created from this snapshot are initialized as if the additional script has already been executed. As the Atom team has shown, using a custom start-up snapshot can significantly boost start-up performance.

To quote the Atom article:

The tricky part of using this technology, however, is that the code is executed in a bare V8 context. In other words, it only allows us to run plain JavaScript code and does not provide access to native modules, Node/Electron APIs or DOM manipulation.

To work around this restriction, electron-link goes great lengths to make sure native functions (backed by C++ functions) are not included in the snapshot, and are instead loaded lazily. V8's serializer simply does not know how to serialize these native functions. Instead, native functions are wrapped into helper functions that load them lazily at runtime.

Since our original post about snapshots, the V8 team has continued developing the snapshot API and added many features and improvements. These features include support for serializing native functions. Provided that the backing C++ functions have been registered with V8, the serializer can now recognize and encode native functions for the deserializer to restore later. We’re happy to announce that the work-around in electron-link is no longer necessary.

One caveat remains: the serializer cannot directly capture state outside of V8, for example changes to the DOM in case of Atom. However, outside state directly attached to JavaScript objects via embedder fields (previously named "internal fields") can now be serialized and deserialized through a new callback API. With some work, this feature allows outside state to be put into the snapshot after all.

Some other highlights include:

FunctionTemplate and ObjectTemplate objects can now be added and extracted from the snapshot, so that they do not have to be set up from scratch.

V8 can now run multiple scripts or apply any other modifications to the context before creating the snapshot, as opposed to only a single source string.

Multiple differently-configured contexts can now be included in the same start-up snapshot blob.

To make these features more accessible, we designed a new, more powerful API with v8::SnapshotCreator. The old API is now merely a wrapper around this underlying API. This is how v8::V8::CreateSnapshotDataBlob is implemented:

The new API is available in V8 version 5.7 and later. We hope that these new features will help embedders make even better use of custom start-up snapshot. If you have any questions, please reach out to our v8-users mailing list.